Macular degeneration is an eye disease that it is the leading cause of blindness for those aged 55 and older in the United States, affecting more than 10 million Americans. The macula is located at the center of the retina, and is responsible for the fine detailed vision required for reading, driving a car, and recognizing objects and colors. While peripheral vision is not affected, the loss of visual acuity has a significant impact on the quality of life of the person afflicted.
Two types of macular degeneration are known. The "dry" type represents 85% to 90% of the cases of macular degeneration and is most closely associated with the aging process. The "dry" type of macular degeneration is characterized by the thinning and drying out of the macula, and the formation of small yellow deposits, known as drusen, under the macula. The amount of retinal thinning caused by the drusen directly affects the loss of central visual acuity.
While the "dry" type of macular degeneration is significantly more common than the second type of macular degeneration, the "wet" type can be more devastating. The "wet" type of degeneration progresses extremely rapidly, whereas the "dry" type progresses much more gradually. The "wet" type of macular degeneration is characterized by the formation of abnormal blood vessels (known as subretinal neovascularization), which grow under the retina and macula. Leakage of blood and other fluids from these abnormal vessels cause the macula to bulge or lift up, thus distorting or destroying central vision. Scar tissue frequently forms, resulting in a permanent loss of vision. Such permanent vision loss can occur in a matter of weeks or months.
While the "wet" type of macular degeneration is less common than the "dry" type, it is significant to note that the "wet" type accounts for 90% of all cases of legal blindness.
If this disease is detected sufficiently early, immediate laser surgery can reduce the severity of vision loss associated with the "wet" type of macular degeneration. In surgically treating the problem, a laser is focused on the abnormal blood vessels and used to destroy them, thus sealing the tissue to prevent blood leakage into the eye and to prevent any additional damage to the macula. Already damaged macular tissue cannot be repaired, and the success of such laser treatments depends on destroying the abnormal vascular before excessive damage to the macular tissue has occurred.
However, laser surgery can also lead to the scarring of the macula, and additional vision loss. The abnormal blood vessels are often difficult to precisely target without causing damage to adjacent normal tissue. Various techniques are being investigated to enable more precise targeting of the abnormal blood vessels, and thereby, to reduce collateral damage to healthy tissue. One method uses a high-speed scanning pulsed laser to rapidly acquire sequences of images of the blood vessels underlying the retina, and to identify individual feeder vessels, which can then be accurately targeted for micro-laser coagulation. While this procedure offers the potential for higher precision laser targeting (thus minimizing the amount of unnecessary damage to surrounding healthy tissue), the required equipment is relatively expensive.
Indocyanine green dye has been used to pinpoint abnormal neovascularization beneath the macula. The dye targets and sensitizes the abnormal vessels to help focus laser energy used in some types of eye surgery. However, the intensity of the laser light employed in the process can still cause damage to non-target normal tissue.
PDT techniques show significant potential in treating these eye diseases. In PDT, a light activated compound is administered to the patient and tends to concentrate in the areas of neovascularization. This absorbed compound is then activated by directing a low-power laser light into the patient's eye and onto the neovascularization areas. When activated, the compound undergoes a chemical change, producing free radicals and/or other products that destroy the abnormal tissue. Miravant of Santa Barbara, Calif. is testing a PDT drug called PURLYTIN.TM. (tin ethyl etiopurpurin), while QLT Phototherapeutics of British Columbia, Canada (with its partner Ciba Vision, Atlanta, Ga.) is testing a PDT drug VERTEPORFIN.TM. (a liposomal benzoporphyrin derivative) for treating ocular diseases. PDT can be relatively selective in destroying only the abnormal neovascularization, and the use of relatively low power laser light to activate the PDT drugs minimizes the risk to surrounding healthy macular tissue.
Although PDT shows significant potential in treating macular degeneration, the laser equipment required for carrying out the above-described PDT is again relatively expensive. The demand for an effective treatment of macular degeneration is high (note that over 10 million North Americans suffer from macular degeneration), and expensive equipment can greatly reduce the availability of such treatment. It would be desirable to develop low-cost apparatus, and methods for the use of such apparatus, to provide greater access to PDT treatment of macular degeneration. The use of a non-coherent light source to activate a photoactive compound offers significant cost advantages over the use of a laser light source.